Update:

The better ones would be:

sed 's/^.+(/|&|?)v=([^&]*).*/2/'

awk 'match($0,/((/|&|?)v=)([^&]*)/,x){print x[3]}'

grep -Po '(?<=(/|&|?)v=)[^&]*'

# Saying match / or & then v=

RFC 3986 states:

   URI           = scheme ":" hier-part [ "?" query ] [ "#" fragment ]



   query         = *( pchar / "/" / "?" )

   fragment      = *( pchar / "/" / "?" )



   pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"

   unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"

   sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"

                 / "*" / "+" / "," / ";" / "="

   …

So to be safe use:

 | sed 's/#.*//' | - to remove #fragment part

in front.

I.e.

| sed 's/#.*//' | grep -Po '(?<=(/|&)v=)[^&]*'

SED (2):

echo 'http://www.youtube.com/watch?v=qdRaf3-OEh4&playnext=1&list=PL4367CEDBC117AEC6&feature=results_main' 

| sed 's/^.+Wv=([^&]*).*/1/'

Explanation:

's       

/…/…/    /THIS/WITH THIS/



'substitute/MATCH 0 or MORE THINGS and GROUP them in ()/WITH THIS/



+-------------------------- s    _s_ubsititute

|+------------------------- /    START MATCH

||                    +---- /    END MATCH

||                    | +-- 1   REPLACE WITH - 1==Group 1. Or FIRS low ().

||                    | | +- /   End of SUBSTITUTE

s/^.+Wv=([^&]*).*/1/'

  +++-+-+-+-+-----+-+------- ^        Match from beginning of line

   ++-+-+-+-+-----+-+------- .        Match any character

    +-+-+-+-+-----+-+------- +       multiple times (grep (greedy +, * *? etc))

      +-+-+-+-----+-+------- W       Non-word-character

        +-+-+-----+-+------- v=       Literally match "v="

          +-+-----+-+------- (       Start MATCH GROUP

            +-----+-+------- [^&]*   Match any character BUT & - as many as possible

                  +-+------- )       End MATCH GROUP

                    +------- .*       Match anything; *As many times as possible 

                                      - aka to end of line; as there is no 



         [abc]  would match a OR b OR c

         [abc]* would match a AND/OR b AND/OR c - as many times as possible

         [^abc] would match anything BUT a,b or c



/1/     Replace ENTIRE match with MATCH GROUP number 1.

         That would be - everything between ( and ) - which his anything but "&"

         after the literal string "v=" - which in turn has a non word letter in 

         front of it.



         That also means that no match means no substitution which ultimately result in 

         no change.

Result: qdRaf3-OEh4

Note: If no match entire string will be returned.

(G)AWK:

echo 'http://www.youtube.com/watch?v=qdRaf3-OEh4&playnext=1&list=PL4367CEDBC117AEC6&feature=results_main' 

| awk 'match($0,/(Wv=)([^&]*)/,v){print v[2]}'

Result: qdRaf3-OEh4

Explanation:

In Awk match(string, regexp) is a function that searches for the longest, leftmost, match of regexp in string. Here I have used an extension that comes with Gawk. (see Awk, GAwk; MAwk etc.) that places the individual matches - that is: what is between parenthesis - in an array of matches.

The pattern is fairly like the Perl/Grep one below.

  +-------------------------------------- Built in function

  |    +--------------------------------- Entire input ($1 would have been filed 1)

  |    |                                  etc. (Using default delimiters " "*)

  |    |

  |    |

  |    |  (....)(....) ------------------ Places Wv= in one group 1, and [^&]* group 2.

match($0, /(Wv=)([^&]*)/, v){print v[2]}

                           |   |    | |

                           |   |    +-+---- Use "v" from /, v; v is a user defined name

                           |   |      +---- 2 specifies index in v, which is group from

                           |   |            what is between ()'s in /…/

                           |   |

                           |   +----------- Print is another built in function.

                           +--------------- Group name that one can use in print.

                              

GREP (Using Perl-compatible):

echo 'http://www.youtube.com/watch?v=qdRaf3-OEh4&playnext=1&list=PL4367CEDBC117AEC6&feature=results_main' | 

grep -Po '(?<=Wv=)[^&]*'

Result: qdRaf3-OEh4

Explanation:

-P  Use Perl compatible

-o  Only print match of the expression.

    - That means: Of our pattern only print/return what it matches.

    If nothing matches; return nothing.



          +------- ^    Negate math to - do not match (ONLY as it is FIRST between [])

          |+------ &    A literal "&" character

          || 

(?<=Wv=)[^&]*

|   | |  |  ||

|   | |  |  |+---- *     Greedy; as many times as possible.

|   | |  +--+----- []    Wild order/any order of what is inside []

|   | +----------- v=    Literal v=

|   +------------- W    Non Word character

+----------------- (?<=  What follows should be (mediately) preceded by.

                    ?=Huh, <=left, = =Equals to



So: Match literal "v=" where "v" is preceded by an non-word-character. Then match

anything; as many times as possible until we are at end of line or we meet an "&".



As you can't have "&" in an URL between key/value pairs this should be OK.